Some words have to be clarified : determination and refinement are not synonymous. For succeeding in a structure determination from powder diffraction data, the way is long and difficult before the final step which is the refinement. Everybody knows that this final step is commonly realized by using the famous Rietveld method (1967-1969). In fact, here we are interested in the whole route between an unknown material and a known crystal structure and to the pitfails which may prevent from being able to refine something.

Powder diffraction is an ancient technique. It would be an error to believe that it is obsolete. There is no sign that powder diffraction could disappear, on the contrary. Thanks to a series of spectacular recent developments (being the consequences mainly of progresses in computers technology and softwares), powder diffraction is a tool more essential than ever for characterization in materials science. This method is predominantly selected for a first contact with an inorganic solid state compound.

The content of this hypertext tutorial follows a definite scenario. It is the story of an unprivileged researcher in solid state "physico-crystallo-chemistry" : this poor guy has not easy access to a neutron nuclear reactor nor to a synchrotron. However, he does his best in a "modern" laboratory (as far as the financial suupport is maintained), equipped at least with an automatized conventional X-ray powder diffractometer, with specialized databanks and with unlimited means for calculations (a PC with a Pentium processor is sufficient those days !). We will make the hypothesis that this researcher has just done an original synthesis by any method, or that his job is the characterization of a compound a priori unknown. This scenario will serve as the basis for examination of a large part of the capacities offered by powder diffraction in materials science. Indeed, a structure determination from powder data makes use of the trivial aspects as well as of the more sophisticated ones, it is quite not easy. Also, it is a last chance method : you should not use it if there is another more accurate possibility for determining the structure of your compound. No mathematical formula will be given in this hypertext. If really you follow this tutorial for solving a concrete case, then it will not be too late for examining mathematics behind the methods involved. Concrete examples will illustrate the successive scenario steps.

The key for success resides in a very careful work during the whole process. The people you have to convince that a step has been successfully realized is first yourself. Only one error and you will waste your time considerably in unfruitful efforts. Finally, nothing can be done without good tools. The main softwares used in the tutorial are : EVA-2, CVRAW, DAT2RIT, DMPLOT, ERACEL, TREOR90, DICVOL91, ITO, FULLPROF, SHELX76, SHELXS86/SHELXL93 (now SHELX97), OVERLAP, STRUVIR. They can be replaced by others (GSAS, etc, see CCP14, SINCRIS, the list of Rietveld and related softwares). A list of those which were used with success in at least one ab initio structure determination from powder data is online.

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Copyright © 1997- Armel Le Bail